1. Field of the Invention
The present invention relates to an image display system utilizing an active-matrix-type liquid-crystal panel or the like as a display. More particularly, the present invention relates to an image display system capable of appropriately displaying image signals having different resolution standards.
2. Description of the Related Art
Referring to FIG. 6, an example of a conventional active-matrix-type display panel will be described briefly. As shown in the figure, this display panel comprises a driving substrate 101, a counter substrate 102, and an electro-optic material 103 held between the driving substrate 101 and the counter substrate 102. As the electro-optic material 103, liquid crystal is widely used. A screen section 104 and a peripheral circuit section are integrated on the driving substrate 101. The peripheral circuit section (unnumbered) is divided into a vertical scanning circuit 105 and a horizontal scanning circuit 106. Further, terminal sections 107 for external connection are formed on the upper end of the driving substrate 101. The terminal section 107 is connected to the vertical scanning circuit 105 and the horizontal scanning circuit 106 via wiring 108. Meanwhile, a counter electrode (not shown) is formed entirely on the inner surface of the counter substrate 102. Gate lines 109 along the row direction and signal lines 110 along the column direction are formed on the screen section 104. The gate lines 109 are connected to the vertical scanning circuit 105, and the column signal lines 110 are connected to the horizontal scanning circuit 106. Pixels formed of pixel electrodes 111 and thin-film transistors 112 are integrated in the intersection portions of the vertical scanning circuit 105 and the horizontal scanning circuit 106. The vertical scanning circuit 105 includes a transferring stage for transferring a vertical start signal in sequence which is input from an external source in synchronization with a vertical clock signal which is input similarly from an external source, and selects in sequence the rows of pixels provided on the screen section 104. The horizontal scanning circuit 106 also includes a transferring stage which operates in response to a control signal, such as a horizontal start signal or a horizontal clock signal, which is input from an external source, and distributes image signals supplied from an external source in sequence to the columns of pixels and writes the image signals into selected pixels.
Image signals supplied from an external source have various standards concerning resolution. Examples of standards for television video signals include NTSC and PAL standards. It is assumed that the screen section of the display panel shown in FIG. 6 has a pixel array conforming to the NTSC standard. In cases where video signals of, for example, a PAL standard, are displayed on this display panel, conventionally, scanning line conversion of PAL video signals is performed so as to perform what is commonly called thinning-out driving of pixel rows. For this purpose, a field memory for temporarily storing video signals is required. Further, in cases where video signals conforming to a normal standard such that the aspect ratio of the screen section is determined at 4:3 are displayed on a display panel conforming to a wide standard such that the aspect ratio is determined at 16:9, a method has already been conceived in which aspect conversion from 16:9 to 4:3 is performed by causing both the left and right end portions of the screen section to be displayed in black. Furthermore, in recent years, an active-matrix-type display panel has been applied to displays for computer graphics. In this case also, a personal computer outputs image signals having various resolution standards. However, image signals output from a personal computer are signals previously formed in units of dots. If scanning line conversion similar to that of television video signals is performed in conformance with the resolution standard, what is commonly called moire occurs on the screen. More specifically, if the sampling timing of image signals shifts in terms of phase from the data array in dot units during the scanning line conversion process, phase differences of a relatively large cycle occur in the pixel array of the screen, and these phase differences appear as moire. Therefore, in order to suppress moire, it is necessary to cause the resolution of image signals supplied from the personal computer or the like to coincide with the pixel array provided on the screen of the display panel. In this case, if image signals (e.g., image signals of a VGA standard) having a relatively low resolution are displayed on a display panel designed for image signals (e.g., image signals of a SVGA standard) having a relatively high resolution, pixels to which image signals are not written remain in the peripheral portion of the screen, causing a blank area to occur. In order to improve the appearance of the screen, it is preferable that the blank area which is other than the display area be displayed black. Conventionally, to make such a black display, black display dot data is inserted among the image signals beforehand. For the purpose of performing such image signal processing, a field memory or a frame memory is required in the components of the image display system, causing problems in that the system configuration becomes complex and the cost is increased.